PhD Thesis Projects

Please contact me any time for PhD thesis projcts. Possible projects comprise numerical simulation of evolution of massive stars, nucleosynthesis, supernovae, gamma-ray bursts, first stars, accreting neutron stars, formation of massive, intermediate-mass and supermassive black holes, binary star evolution and formation of binary black holes, ...

Summer Student Projects

The effect of rotation on supernova progenitor stars

Background:
Stars born with eight solar masses or more run through all major nuclear burning phases, forming an iron core that collapses to make a neutron star or a black hole. During the collapse phase, the outer layers may be ejected in a spectacular explosion observed as a supernova, or even a more powerful jet-powered gamma-ray burst. In some rare cases, this terminal explosion may be mediated by rotation, e.g., through formation of an accretion disk around a black hole, or by a rapidly rotating, highly magnetised neutron star. On the other hand, massive stars are know to be born with fairly high initial rotation rates. The effect of this rotation is important not only at the end of the star's life, but can also change how the star evolves. For example, the limiting minimum mass for making supernovae or the transition from making neutron stars to making black holes can change significantly.

Project Outline:
The goal of this project is to study the effect of rotation on the evolution of a massive star in detail. For this, you will run numerical simulations of stellar evolution including rotation. You will study how the evolution of the star changes in detail as the initial rotation rate increases, and how this changes the final outcomes. You may also study the effect that the individual rotationally-induced instabilities have on internal mixing of composition and angular momentum transport, and the effect of mass loss on the rotation.

Perspective:
Understanding the details of how rotation impacts the evolution of massive stars remains one of the major uncertainties in our understanding of the final fates of massive stars. Recent gravitational wave observations by LIGO are also sensitive to the spin of the neutron stars and black holes at the time of merger in a binary star system, and LIGO may be able to see the spin of the stellar core during a collapse of a close-by supernova.

Project Details:

References

The Nature of the First Stars.

Background:
After the Big Bang it took about 300,000,000 yr before the first stars would form - now some 13,000,000,000 yr ago. Unfortunately, we can no longer observe these stars today directly, even with our best telescopes. But there is still some "fossil" record of them left behind, preserved in the oldest stars in our galaxy we can observe, dating back to pre-galactic times. When the first stars exploded as supernovae, their ashes were dispersed and the next generation of stars formed, incorporating some of these supernova debris. We can now measure these abundance patterns in those old stars (in particular done by astronomers in Australia, including Physics Nobel Prize Laureate Brian Schmidt, and using telescopes here in Australia and the largest telescopes in the world in Hawaii and Chile). In fact, we have a rapidly growing catalogue of them. To some extent, hence, the abundance patterns are similar to a genetic fingerprint that allows to identify the parents.

Project Outline:
The goal of this project is to identify the "parents" of these old stars in our galaxy, i.e., find out about the now "extinct" first stars in the universe - what their properties where, how they lived and died - and even how many parents there were, how different or alike they were. For this we have to create a data base of predicted abundance patterns of supernovae from the first generation of stars (picture left/above) to compare with the elements observed in the old stars. Your work will be help creating the data base and to compare with the most recent data we have and develop some analysis tools to allow you do this task.

Perspective:
This work will be part of a larger project to understand the nature of the first stars. Depending on findings, extension to this work may lead to a short publication or letter in a refereed journal. Extension to 3rd year project or honours thesis may possible, or lay the ground work for such a project in this field.

Project Details:

References

The Neutrino HR Diagram.

Background:
For millennia mankind has observed stars by eye, later by telescopes - optical, radio, X-rays, etc., but always if the form of electromagnetic radiation. Accordingly, we typically record and classify the evolution of stars as function of their surface properties, i.e., their surface temperature and their luminosity (the Hertzsprung-Russell (HR) Diagram). But stars do not just radiate light, but also emit neutrino radiation. In fact, even the sun emits about 7% of all its energy in the form of neutrinos. So far, however, the only stars we have seen as neutrinos are the sun and the supernova 1987 A - detecting neutrinos is very hard, but technology is improving, so it is good to make predictions what we may see, or should see, some day. In fact, evolved stars may shine 10,000,000,000 times more brightly in neutrinos at the end of their life as they shine in visible light - so observing a star in neutrinos tells us that is very close to death, maybe weeks to hours - and give us an early warning about an impending supernova. Additionally, neutrinos not only have an energy spectrum like electromagnetic radiation, but they also have "flavour" as a "colour".

Project Outline:
The goal of this project is produce the counterpart of such an HR diagram but for neutrinos and make graphical representations. How would stellar evolution tracks look? Maybe including supernovae? How would snapshots of different astronomical objects like star clusters or galaxies look?

Perspective:
This work will be part of a larger project to understand the nature of the first stars. Depending on findings, extension to this work may lead to a short publication or letter in a refereed journal. Extension to 3rd year project or honours thesis may possible, or lay the ground work for such a project in this field.

Project Details:

The Most Common Thermonuclear Explosions in the Universe: Type I X-ray Bursts.

Background:
The lifetime of a star is highly dependent upon its initial mass: the sun will shine for a total of about 10 billion years; for a star 25 times the mass of the sun this reduces to a mere 10 million years. When a star of about ten solar masses or more reaches the end of its life, it may explode as a supernova and leave behind a neutron star or black hole. But many stars are not single. Instead, they may have a close companion star. These are called binary star systems. In some cases the secondary star has much lower mass than the primary star and hence way outlive its more massive partner. If the two stars are close enough and the secondary star expands as it evolves, it may transfer mass to the remnant of the primary star. The mass then spirals inward toward the star in an accretion disk emitting X-rays. In case the remnant is a neutron star, the accreted material accumulates on the surface of the neutron star. When the accreted layer gets big enough, it can ignite in a bright thermonuclear flash incinerating the accreted layer of nuclear fuel. We can observe such flashes as Type I X-ray bursts throughout the entire galaxy. The bursts last just seconds and recur on a time scale of hours to days. Considering there is some hundred of such systems active in our galaxy, this makes them the most common thermonuclear explosion to occur in nature.

Project Outline:
The goal of this project is to model such Type I X-Ray bursts in a special kind of system: stars in which the companion is a helium white dwarf stars, hence the accreted material is mostly pure helium. These are much easier to model than system that accrete material which also contains hydrogen. You will be using a hydrodynamic code to simulate such bursts for a variety of conditions, study their behaviour, and compare to observational data.

Perspective:
This project will get you started on understanding thermonuclear explosions on neutron stars. An extension to 3rd year project or honours thesis is possible and welcome. Depending on progress this work should then lead to a scientific publication in a refereed journal.

Project Details:

Honours Student Projects

What is in the purest stars?

Supervisors:
Alexander Heger (MoCA)
Kais Hamza (Maths)
Mike Bessell (RSAA/ANU)

Background:
After the Big Bang it took only a few minutes to synthesise the primordial composition of the universe, essentially only hydrogen and helium, with traces of lithium and negligible amounts of everything else. All heavier elements were synthesised in stars. From the Big Bang it would take a few hundred thousand years before atomic nuclei and electrons combine to neutral atoms and molecules. And few hundred million years before the first stars formed. This first generation of stars forged the first heavy elements in the universe and released them back into outer space when these stars exploded as supernovae. The material was diluted with the vast amounts of gas left by the big bang, and then incorporated into the next generation of stars. This way the universe became increasingly enriched in heavy elements as we find them in the crust of the earth, to make up planets, and being necessary to life. The very first generation of stars is thought to be quite short-lived, and all of them are gone by now. The second generation would only have very small trace of the ashes of the first generation of stars, and being much longer-lived, we can find them in our galaxy today. The ratio of elements in these ashes provides important clues as to the nature of the elusive first generation of stars. But for many of the elements the abundances are so small that only upper limits can be determined. Yet even these upper limits provide important clues that we want to use.
Recently the most iron-poor star known was discovered by Australian Astronomers. Only for a hand full of elements abundances could actually be measured, while for many other just upper limits could be estimated. Currently there is, however, no good statistical model in Astronomy how to best estimate these upper limits and how to use these upper limits to constrain the nature of the first stars.

Project Outline:
The goal of this project is to derive upper limits for abundances of chemical elements and confidence level for these upper limits provided observational and model data. Abundances of chemical elements are determined by matching spectral lines from atomic and hydrodynamical stellar atmosphere calculations to data taken by the Hubble Space Telescope and by some of the world's largest telescopes in Chile and on Hawaii. A possible approach would be to simulate observations given the model data and the same level of noise, to then determine the detectability and confidence levels. A second goal is to develop a model to constrain theoretical data for production of heavy elements by the first stars.

References:
http://adsabs.harvard.edu/abs/2014Natur.506..463K
http://adsabs.harvard.edu/abs/2015ApJ...806L..16B

Helium-rich massive stars in Globular Clusters

Supervisors:
Alexander Heger
Amanda Karakas

Background:
Globular clusters are some of the more spectacular yet mysterious components of galaxies. They are usually old objects with some 100,000 old stars. Their exact origin and formation is not known, however. Whereas it was once assumed they would form monolithically in one star formation event from just a single chemically homogeneous gas cloud, modern astronomical observations allowed us to identify at least two, sometimes more, distinct stellar populations within most of them, visible in the colour-magnitude diagram. These different populations are chemically very similar in some chemical elements, but distinctly different in others. One of the key differences is in their helium enrichment. This is what changes the location of the low-mass stars in the colour-magnitude digram, as observed. But how does it affect more massive stars, especially those that make supernovae?

Project Outline:
The goal of this project is to model the evolution massive supernova progenitor stars, of globular cluster chemical composition, but with varying degrees of helium enrichment. The project will use a modern stellar evolution code to model evolution, nucleosynthesis, and supernova explosions of massive stars. Depending on project progress, an extension could be to compare to models of rotating massive stars and their nucleosynthesis. The results will be compared to the observational data.

How fast do old stars rotate inside?

Supervisors:
Alexander Heger
John Lattanzio
Paul Cally

Background:
The Kepler satellite mission became famous for finding hundreds of new planets around stars. To do so, it had to monitor them in minute detail, including all variations and oscillations of the star. Amazingly, the data was good enough to use seismology, similar to what we do on earth to determine its interior structure, or for the sun ("helioseismology"), to determine the interior structure and rotation rate of evolved - old - stars that approach the end of their life. For the Sun, we can easily observe how fast it rotates on the surface, and we know that the same rotation rate, on average, is maintained all the way to the centre. But for all other stars, the interior rotation rate was not known to date. But now we have observations. The big questions is whether our current model for transport of angular momentum and stellar evolution is good enough to explain this data. Having this data is a unique new opportunity to test physical models for the action of rotation inside stars.

Project Outline:
The goal of this project is to model the evolution of a star like the long past its current age until the Red Giant and Horizontal Branch evolution phases. The project will use a modern stellar evolution code, and the code would also be modified to test different physics models for the action of magnetic dynamos and hydrodynamic instabilities due to rotation. The results will be compared to the observational data.

References:
2005ApJ...626..350H
2002&A...381..923S

Weak Helium Flashes in Accreting Neutron Stars.

Supervisors:
Alexander Heger
Duncan Galloway

Background:
Many stars are not single stars like the sun, but are born as binary stars, two stars in a close orbit about each other. If one of the stars is "massive," more than about ten times the mass of the sun, it may end is life in a supernova and leave bind a neutron star. In some cases where the other star in the system is of lower mass, and hence loves longer, the orbit could be tight enough that as this star evolves it swells up enough to transfer mass to the neutron star. The accreted mass accumulates in a layer at the surface, and usually starts some burning immediately (hot CNO cycle). When the layer gets thick enough, it may burn in a brief powerful flash burning material all the way to quite heavy material. This is observed as a Type I X-ray burst. If the accretion is very slow, however, the layer may be so cool, the burning does not start immediately, and when it starts, it may just start hydrogen burning, then subside. Only after several of these weak flashes, a more powerful burst might result.

Project Outline:
We will use a hydrodynamic stellar evolution code including an extended nuclear reaction network to follow the accretion and burning flashes. The goal is to explore the regime of weak flashes and where they occur and what is their behaviour as a function of neutron star properties and accretion rate and composition (originating from the companion star). A possible extension of the project is to implement the physics of gravitational settling in the present code.

References:
2004ApJS..151...75W
2010ApJ...725..309P
2003ApJ...599..419N
2007ApJ...654.1022P

The Fate of the Biggest Stars

Supervisors:
Alexander Heger
Bernhard Mueller
Anthony Lun

Background:
One of the biggest puzzles in understanding the formation and structure of Galaxies are the huge black holes in their centres. Some of them have a billion time the mass of the sun, even when they are only a tenth of their percent age. One, highly speculative, theory is that they may start as the collapse of supermassive stars of maybe a million times the mass of the sun, from the first, or very early, generation of stars that precede the first galaxies ("pre-galactic stars"). Whereas supermassive stars of primordial composition either undergo hydrostatic burning or collapse to black hole, stars that have some enrichment in material from a previous generation of stars may instead explode, probably the most powerful explosions in the universe other than the big bang itself. But where exactly are the boundaries between explosion, collapse, and hydrostatic burning?

Project Outline:
The goal of this project is to find the boundaries between hydrostatic burning, thermonuclear explosion, and collapse to a black hole for supermassive stars, i.e., stars of some 100,000 times the mass of the sun. You will use a hydrodynamic stellar evolution code that includes thermonuclear burning and post-Newtonian corrections for general relativity for non-rotating stars. The simulations will start with stars of different initial mass and different initial composition and will follow the early evolution of supermassive stars until they either collapse, explode, or reach hydrostatic burning. A possible extension of the project is to modify the stellar evolution code to include post-Newtonian corrections for rotating stars.

References:
2011JPhCS.314a2077M
2012ApJ...749...37M
2001ApJ...552..459H
1986ApJ...307..675F

The Formation of Supermassive Black Holes

Supervisors:
Alexander Heger
Bernhard Mueller
Anthony Lun

Background:
One of the biggest puzzles in understanding the formation and structure of Galaxies are the huge black holes in their centres. Some of them have a billion time the mass of the sun, even when they are only a tenth of their percent age. One, highly speculative, theory is that they may start as the collapse of supermassive stars of maybe a million times the mass of the sun, from the first, or very early, generation of stars that precede the first galaxies ("pre-galactic stars"). Models of supermassive stars of primordial composition suggest that these either undergo hydrostatic burning or collapse to a black hole. But these stars do not form at once, but rather start from a small core that accretes mass at a high rate, as current simulations of early start formation suggest, until a super-massive star is built up.

Project Outline:
The goal of this project is to find how such stars with primordial composition and high accretion rates evolve and approach the point of collapse to a supermassive black hole, as a function of this accretion rate. And, in particular, what the mass of the star is by the time it collapses, i.e., what is the mass of the black holes formed. For example, is there an upper mass limit, and is this different from the one obtained for stars with a given fixed initial mass (see other project).

References:
2011JPhCS.314a2077M
2001ApJ...552..459H
1986ApJ...307..675F
2013ApJ...777...99W

Constraining Supernovae Properties by their Nucleosynthesis.

Supervisors:
Alexander Heger
Bernhard Mueller

Background:
Most heavy elements from oxygen to iron are dominantly made by the deaths of massive stars as supernovae. Whereas fully understanding such core collapse supernovae requires multi-dimensional simulations including complicated and expensive radiation transport physics, there is some progress in developing simpler approximation formulae for these supernovae given the structure of the star at the time of its death. Depending on the explosion properties, supernovae synthesise and eject elements in different proportions, which can be used as a diagnostic of the explosion model.

Project Outline:
For this project you will use an analytic model for supernova explosions and their energies to simulate the nucleosynthesis of these stars. The result is to be compared to the abundance patterns - elemental and isotopic - that we find in the in the universe today, in the sun, and on earth. The goal of the project is to constrain the properties of the analytic supernovae model in its ability to reproduce the observed data.

References
2012ARNPS..62..407J
arxiv.org/abs/1409.0540
2002RvMP...74.1015W
2002ApJ...576..323R

Supernovae Making Neutron Stars or Black Holes?

Supervisors:
Alexander Heger
Bernhard Mueller

Background:
When a massive star reaches the end if its life, the core collapses into a neutron star or, possibly, a black hole. In many cases, at first a shock is launched moving outward, ejecting the outer layers of the star. But there may not be enough energy to eject the entire core, or there can be hydrodynamic interactions in the envelope that push some of the matter onto the central object. How much of the material falls back will determine the final mass of the compact remnant that is left behind. If the mass exceeds the maximum mass for a neutron star, it will collapse to a black hole.

Project Outline:
For this project you will use an analytic model for supernova explosions and their energies to simulate the explosion of these stars. You will then use a one-dimensional hydrodynamic code modified for proper inner boundary conditions, to simulate the dynamics of the explosion and how much mass is ejected or fall back. This will allow you to estimate the remnant mass (some of the rest mass is carried away by neutrinos). Using a range of supernova progenitor models, you can make perditions about the distribution of neutron star and black hole masses.

References
2008ApJ...679..639Z
2012ARNPS..62..407J
arxiv.org/abs/1409.0540
2002RvMP...74.1015W
2003ApJ...591..288H

Previous Projects (Monash)

Honours Projects

Supercomputer Simulations of Superbursts.

Supervisors:
Alexander Heger
Duncan Galloway
Yuri Levin
Bernhard Mueller

Background:
Many stars are not single stars like the sun, but are born as binary stars, two stars in a close orbit about each other. If one of the stars is "massive," more than about ten times the mass of the sun, it may end is life in a supernova and leave bind a neutron star. In some cases where the other star in the system is of lower mass, and hence loves longer, the orbit could be tight enough that as this star evolves it swells up enough to transfer mass to the neutron star. The accreted mass accumulates in a layer at the surface, in some cases periodically igniting in a flash observed as Type I X-ray burst. These bursts leave behind ashes that accumulate - very few things ever escape from the surface of a neutron star. When the ashes layer gets quite thick, it may burn in a powerful flash called superburst, fuelled by the carbon left behind in the ashes.

Project Outline:
You will use a two-dimensional hydrodynamic code to simulate the thermonuclear runaway and the explosion of such superbursts. You will follow the onset of nuclear burning through the thermonuclear runaway to the formation of a shock wave that travels to the surface of the stars where then a superburst can be observed. The question to answer is how character of the burst and the transitions depend on the neutron star properties and of the accreted layer.

References:
2004ApJS..151...75W
2003ApJ...599..419N
2001ApJS..133..195Z
2012ApJ...752..150K

How the oldest stars known were made.

Supervisors:
Alexander Heger
Aldeida Aleti
Steven Mascaro

Background:
After the Big Bang it took about 300,000,000 yr before the first stars would form - now some 13,000,000,000 yr ago. Unfortunately, we can no longer observe these stars today directly, even with our best telescopes. But there is still some "fossil" record of them left behind, preserved in the oldest stars in our galaxy we can observe, dating back to pre-galactic times. When the first stars exploded as supernovae, their ashes were dispersed and the next generation of stars formed, incorporating some of these supernova debris. We can now measure these abundance patterns in those old stars (in particular done by astronomers in Australia, including Physics Nobel Prize Laureate Brian Schmidt, and using telescopes here in Australia and the largest telescopes in the world in Hawaii and Chile). In fact, we have a rapidly growing catalogue of them. To some extent, hence, the abundance patterns are similar to a genetic fingerprint that allows to identify the parents.

Project Outline:
The goal of this project is to identify the "parents" of these old stars in our galaxy, i.e., find out about the now "extinct" first stars in the universe - what their properties where, how they lived and died, and in particular, how many parents there were, how different or how alike or different they were. You will be using a data base of predicted abundance patterns of supernovae from the first generation of stars to compare with the elements observed in the old stars. A particular goal will be to assess the likelihood of multiple stars fits as compared to the extra degrees of freedom introduced. This could be done, e.g., using metricates like the minimum message length or Bayesian networks. You may also develop and employ new methods for optimal pattern matching using as much of the information available from observational data as possible.

References
Nature, 506, 7489 (2014)
2010ApJ...724..341H
2002ApJ...567..532H

Supermassive Stars: Explode or Die?

Supervisors:
Alexander Heger
Yuri Levin
Anthony Lun

Background:
One of the biggest puzzles in understanding the formation and structure of Galaxies are the huge black holes in their centres. Some of them have a billion time the mass of the sun, even when they are only a tenth of their percent age. One, highly speculative, theory is that they may start as the collapse of supermassive stars of maybe a million times the mass of the sun, from the first, or very early, generation of stars that precede the first galaxies ("pre-galactic stars"). Whereas supermassive stars of primordial composition either undergo hydrostatic burning or collapse to black hole, stars that have some enrichment in material from a previous generation of stars may instead explode, probably the most powerful explosions in the universe other than the big bang itself. But where exactly are the boundaries between explosion, collapse, and hydrostatic burning?

Project Outline:
The goal of this project is to find the boundaries between hydrostatic burning, thermonuclear explosion, and collapse to a black hole for supermassive stars, i.e., stars of some 100,000 times the mass of the sun. The student will use a hydrodynamic stellar evolution code that includes thermonuclear burning and post-Newtonian corrections for general relativity for non-rotating stars. The simulations will start with stars of different initial mass and different initial composition and will follow the early evolution of supermassive stars until they either collapse, explode, or reach hydrostatic burning. One possible extension of the project is to modify the stellar evolution code to include post-Newtonian corrections for rotating stars; another extension could be to follow the neutrino signal of collapsing stars and the neutrino-induced nucleosynthesis in the envelope of the star, as well as a possible explosion due to the mass carried away by the neutrinos.

References:
2011JPhCS.314a2077M
2012ApJ...749...37M
2001ApJ...552..459H
1986ApJ...307..675F

Summer Vacation Projects

The Making of the First Heavy Elements in the Universe.

Background:
After the Big Bang the universe consisted basically of just hydrogen and helium; all heavy elements were forged later, by the first generation of stars. Yes the nucleosynthesis ashed of this first generation of stars were incorporated in the next generation of stars and into the first small proto-galaxies and galaxies that ever formed. Later, the material, possibly re-ejected from the second generation of stars, though somewhat diluted, gets ever re-cycled in subsequent generations of stars to the present day. Observing old and very old stars, some of them having lived almost as long as the age of the universe, but also some of the more recently formed ones, in the in our and neighbouring galaxy today and analysing their composition from their spectra, we can try to reconstruct the galacto-chemical history of the universe, including its beginning from seeds for the very first stars.

Project Outline:
To study the chemical evolution of galaxies and of the universe from its beginning, we need to know stars produce and eject, in form of winds and supernovae - of a population of stars - as a function of time. The reason is that both stellar lifetimes and element production vary with initial mass of the star.
Your task will be to develop a tool that allows to determine the nucleosynthesis contributions from the first generation of stars for the use of galacto-chemical evolution models. What is already available is a library of models of supernovae from the first generation of stars for varying explosion energies. We will use a model to determine the best choice of explosion energies and hydrodynamical instabilities in these supernovae. This model, in fact, was developed in collaboration with two previous Summer Vacation Scholars (second reference). If the project goes well, it may be extended to us the new data in galacto-chemical model codes yourself, during the Summer Vacation Scholarship or in a follow-up project.

Perspective:
Depending on progress this work may lead to a short publication or letter in a refereed journal. Extension to 3rd year project or honours thesis is possible.

Project Details:

References

The Cosmic Forge: How the Most Powerful Supernovae Make Heavy Elements.

Background:
When massive stars of about ten times the mass of the sun or more reach the end of their life, their centre collapses to a neutron star or a black hole. At the same time, a supernova shock front may be launched that that disrupts the stars such that much of the nucleosynthesis products the star has made throughout its life are ejected, to finally make new stars or become parts of planets and form the basis for life. As the shock travels through the star, it also causes explosive burning in the ejected material. What exactly is being made and what is ejected, however, strongly depends on the energy of the explosions. From observations we know that some the biggest massive stars may explode with tremendous amount of energy, maybe ten times as much as a "regular" supernova. the prized question is what exactly is being synthesized and ejected, in order to allow us reconstructing the history of our galaxy and the chemical evolution of the universe.

Project Outline:
The goal of this project is to study the nucleosynthesis of these powerful supernovae, so-called "hypernovae" using a hydrodynamical code that includes a large nuclear reaction network.
Your task will be to study the how the nucleosynthesis products of the star change as the explosion energy is adjusted. The results can be compared to the ratio of isotopes and elements as we find them in the milky way, in the sun, and on earth, helping you to constrain which parameters are physically realistic and necessary to explain observational data. Simply put, this way you can constrain the properties of dying stars, simply put, just by looking at the composition of a piece of rock you may find in your backyard.

Perspective:
Depending on progress this work may lead to a short publication or letter in a refereed journal. Extension to 3rd year project or honours thesis is possible.

Project Details:

The Last Three Minutes: How Does a Star Look Inside Just Before It Dies?

Background:
Stars more massive than about ten solar masses run through the full sequence of thermonuclear burning phases to form an iron core in their centre. Which the core collapses, it is still surrounded by by layers of increasingly lighter material as one goes further out. This surrounding material start burning violently as centre contracts and eventually collapses. This rapid burning causes violent convection, which in turn causes fluctuations in the density and velocity field of the infalling matter. These deviations from spherical symmetry can have significant influence on the dynamics of the collapse, the supernova mechanism, and likely how big a kick the neutron star receives when it is born. These fluctuations may play critical role in our understanding of how supernovae work in the first place. It may even decide whether a star explodes as a powerful supernova or collapses to a black hole instead.

Project Outline:
The goal of this project is to study the these density fluctuations just before the star dies. For this purpose you will simulate the last few minutes of evolution of star up to the onset of iron core collapse for a range of massive stars. You will try to understand the landscape of Mach number and Bulk Richardson number in supernova progenitors of the material that determines the supernova properties using estimates based on an analytic model.

Perspective:
Depending on progress this work will make a significant contribution to a current work on understanding how supernovae explode and which stars explode. Depending on progress and outcome, you work is intended to become part of a resulting publication. An extension to 3rd year project or honours thesis is possible.

Project Details:

The Cosmic Forge: Supernovae and their Nucleosynthesis.

Background:
When massive stars of about ten times the mass of the sun or more reach the end of their life, their centre collapses to a neutron star or a black hole. At the same time, a supernova shock front may be launched that that disrupts the stars such that much of the nucleosynthesis products the star has made throughout its life are ejected, to finally make new stars or become parts of planets and form the basis for life. As the shock travels through the star, it also causes some explosive burning in the innermost material that changes the composition of the material that is actually ejected. The tremendous neutrino flux emanating from the newly born neutron star additional contributories by transforming or splitting some of the nuclei. A huge uncertainty, however, is with how much energy the star actually explodes, even if we know its entire structure prior to the supernova explosion. And this, of course, changes which elements the star forges and how much of each it expels.

Project Outline:
The goal of this project is to study the nucleosynthesis of supernovae using an analytic model for supernova explosions. This model makes some assumptions about certain physical parameters of the explosion. Your task will be to study the how the nucleosynthesis products of the star change as the model is adjusted. The results can be compared to the ratio of isotopes and elements as we find them in the milky way, in the sun, and on earth, helping you to constrain which parameters are physically realistic. This way you can constrain the properties of dying stars, simply put, just be looking at the composition of a piece of rock you may find in your garden.

Project Details:

The Genes of the Oldest Stars.
(The Stellar Jurassic Park)

After the Big Bang it took about 300,000,000 yr before the first stars would form - now some 13,000,000,000 yr ago. Unfortunately, we can no longer observe these stars today directly, even with our best telescopes. But there is still some "fossil" record of them left behind, preserved in the oldest stars in our galaxy we can observe, dating back to pre-galactic times. When the first stars exploded as supernovae, their ashes were dispersed and the next generation of stars formed, incorporating some of these supernova debris. We can now measure these abundance patterns in those old stars (in particular done by astronomers in Australia, including Physics Nobel Prize Laureate Brian Schmidt, and using telescopes here and the largest telescopes in the world in Hawaii and Chile). In fact, we have a rapidly growing catalogue of them. To some extent, hence, the abundance patterns are similar to a genetic fingerprint that allows to identify the parents.

The goal of this project is to identify the "parents" of these old stars in our galaxy, i.e., find out about the now "extinct" first stars in the universe - what their properties where, how they lived and died - and even how many parents there were, how different or alike they were. We want to use a genetic algorithm (an optimization method) to find a match and combination of "ashes" from theoretical models in a large data base containing a wide variety of stellar models and supernova and compare to observational data. The student's task will be to develop a code using a genetic algorithm as optimization method to find out which theoretical data (relative abundances of chemical elements) best matches the our best current observations. Some basic programming experience would be advantageous, some mathematical skills are required, but you definitively need to bring the willingness to learn.

Are you ready to recreate "The Stellar Jurassic Park"?

Supernova Archaeology.

For some supernova remnants like Cas A we now have detailed observational data that give is distance from the center and velocity w/r the observer. Usually astronomers assume homologous expansion of the ejecta - velocity just scales linearly with distance from the center. Using this relation, various structures in the supernova have been recovered, in 3D, some of them rather surprising, like planar "walls" of material. But are these structures real or just an artifact of the reconstruction procedure? The goal of this project is explore different velocity distributions of the ejecta and how they would appear when reconstructed using the assumption of homologous expansion mentioned above. Assume, for example, some ejecta would come out as a bubble on one side, as we find in supernova simulations. How would such a structure appear? Using and developing 3D visualization as well as data from the literature and publications are essential parts of this project.

Previous Projects (UMN) (in progress)

OpenCL Sparce Matrix Solver
(computer science)		 Implement space matrix solver for CUDA (nVidia graphics card) or OpenCL to accelerate nuclear reaction network solver on current and future computer hardware.
Lowest M/Z ONeMg WD
(astrophysics)		 Determine the minimum mass for making ONeMg white dwarf stars at the lowest metallicities. How does stellar evolution change for very- and ulta-metal poor stars?
Lowest M/Z supernovae
(astrophysics)		 Determine the minimum mass for core collapse supernovae at the lowest metallicities. How does stellar evolution change for very- and ulta-metal poor stars?
Heavy Metal Snowstorms
(theoretical/astrophysics)		 1D simulations of instabilities inside accreting neutron stars due to phase separation. This project may also require some theoretical work, maybe some background in condensed matter physics.
Type I X-ray bursts
(astrophysics)		 Multi-D simulations of mixing and burning in the thermonuclear runaway of a thin layer of accreted material on neutron stars in a binary star system.
Consistent data mapping
(computational fluid dynamics)		 Implement algorithm to conservatively map data from 1D Lagrangian coordinate system to multi-D Eulerian coordinates as initial conditions for numerical simulations. Use multi-D simulations to assess quality of mapping.
More than very massive stars
(astrophysics)		 Study the evolution of stars that collapse to black holes beyond the "classical" limit of very massive stars . How does the evolution of stars change in that mass range?
Evolution of supermassive stars
(astrophysics)		 Determine the mass limits and dependence on initial conditions, for which supermassive stars collapse, and which still burn, and for how long, before they collpase.
Supermassive Supernovae
(astrophysics)		 Determine mass limits as a function of metallicity as well as dependence on initial conditions, for which supermassive stars explode.
Evolution of the Sun
(astrophysics, thesis/directed research)		 Numerical simulations (1D) of the evolution of the Sun. Try to reproduce the current Sun at the current age, and follow the evolution to late times. How will the sun end it life?
AGB stars
(astrophysics)		 Follow the evolution of and intermediate mass stars to late times. Simulate nucleosynthesis in these stars.
Supernova remnant masses
(astrophysics)		 Perform numerical simulations of fallback ejecta after a supernova explosion to determine the mass and type of the remnant. Is a neutron star or a black hole formed?
IMF of the First Stars
(astrophysics, graduate student)		 Match nucleosynthesis patterns of observed metal-poor stars to nucleosynthesis predictions from stellar models. Try to deduce what stellar masses are the best fit to the observed abunacne patterns. Develop a tool and plotting to also incorporate isotopes.
Galactochemical Evolution
(astrophysics, graduate student)		 Determine the evolution of different components of nucleosynthesis products (r-, s-, p-process, etc.) as a function of metallicity. Combine observational data for the evolution of different elements with that of different components for isotopes from nuclear data and nucleosynthesis studies. Construct isotopic galactochemical history of the universe.

--- Alexander Heger